1. Field of the Invention
The present invention relates to a digital printing device operating by spraying ink onto a substrate, which may have either an “open” in other words absorbent surface such as for example paper or cardboard, or a “closed” in other words non-absorbent surface such as for example some plastic materials or glass. The principle of this so-called ink jet technology consists in spraying fine ink drops onto the substrate in a matrix based pattern, so as to print characters or graphics from digital data.
2. Description of Related Art
This printing principle has been in use since about the 1970s in respect of black and white and since the 1980s in respect of color printing. Applications exist in particular in the field of high-speed printing, low-cost color printers for personal computers, or industrial printing on a variety of substrates. The present description applies to printing on any substrate, <<open>> or <<closed>>, for example by a machine for customizing plastic cards or other portable objects, but it is obvious that such an invention may also apply to a number of other cases.
In the case of industrial machine printing, printing is carried out by a printhead including one or more electronically controlled print nozzles supplied by a reservoir containing ink in liquid form. These nozzles are able to operate according to a “Drop or Dot On Demand” principle, these drops being released for example by piezoelectric effect. Other systems operate according to the “deflected jet” principle, whereby an ink jet is propelled permanently towards the substrate, and whereby electrically charged electrodes deflect this jet into a gutter at times when printing a dot is not required. The unused ink is recovered and sent back to the ink reservoir.
These components are combined in a print module which moves over the print substrate so as to cover the whole required print surface while being connected to a fixed control bay. This unit may constitute one of the stations of a machine or a line of machines for producing and customizing plastic cards or other portable objects.
On machines intended to work at high speed, for example more than 2000 cards per hour, the print module moves at a speed which is sufficiently high for the liquid ink contained in the reservoir to present significant splashing or agitation. Such agitation may give rise to a number of drawbacks: a variation in ink height and therefore in pressure and therefore in size of the ink drops deposited on the substrate, formation of bubbles in the reservoir, lack of ink coming from the outlet hose even if the reservoir is not empty, difficulty in getting a valid measurement of the ink level in the reservoir.